Sustainable Approaches to Corrosion Protection in Reinforced Concrete: A Critical Review of Green Inhibitors, Surface Treatments, Self-Healing Systems, Nanomaterials, Smart Coatings, and Life-Cycle-Based Design

Corrosion of steel reinforcement is still the main reason reinforced concrete structures fail before the end of their planned service life. Every early repair costs money, and it also adds new cement, steel, and emissions. This review examines protection methods that can extend service life while lowering environmental impact. It covers green and nano-enabled corrosion inhibitors, hydrophobic and polymer-modified surface treatments, autogenous, bacterial, and capsule-based self-healing systems, nanomaterials, and stimulus-responsive smart coatings. It also covers supplementary cementitious materials, limestone calcined clay cement, and alkali-activated binders as the first barrier against chlorides and carbon dioxide. The review then links these materials to durability-based design, probabilistic service-life models, life cycle assessment, machine learning, and sensor-based monitoring. Three findings stand out. First, most green inhibitors and smart coatings have been tested in pore solution or in short accelerated tests, and long-term data in real concrete are scarce. Second, a gain in chloride resistance only counts as a sustainability gain when it is expressed per year of service life, including maintenance. Third, data-driven models predict chloride transport well inside their training data, but few of them respect physical limits or report uncertainty. We grade each method by the evidence behind it, from pore-solution tests to long-term field use, and by its technology maturity. Most of the newer methods have not yet left the laboratory. A worked example shows that holding the chloride diffusion coefficient at its 28-day value understated the time to corrosion initiation of fly ash and slag concretes by a factor of about 4 to 7. Here, sustainability refers mainly to climate impact, expressed as global warming potential per year of service life. We propose a design loop with two decision gates. Unlike existing service-life or life cycle frameworks, it checks reliability and climate impact together, and it gives credit only to methods with measured inputs in mortar or concrete. It connects binder choice, added protection, probabilistic service-life prediction, and life cycle metrics, and it updates itself with monitoring data. The review closes with a ranked list of research gaps and practical steps for researchers, designers, and asset owners.